Only render what's visible from camera's view when creating multi-layer EXR?
Separate the object you want to render by layer.


Place the render object on layer 2. (e.g. Torus)
Place the masking objects (all other objects) on layer 1. (e.g. Cube)
Make both layers visible in the Scene's layers.
Set the Layer to layer 2 in the render layers tab.
Set the Mask Layer to layer 1 (or more) in the render layers tab.


The objects on layer 1 will now act as a mask.





Compositing in Photoshop

Sadly, stacking these image on top of each other messes up the premultiplication / alpha / hard to explain.

Here's how to composite these two layers in Photoshop.

Basically, we add all akphas together, add all colors together, then use the alphas as a mask.


Create layers containing only the alphas.
Set the lowest alpha layer to Normal blending mode, the others to Add.
Merge the alpha layers creating an alpha for all layers together (you could also render this from Blender).





Create a group (folder) for each layer. In each group add a black solid layer below the imported image.
Set the lowest group's blending mode to Normal and all other group's blending modes to Add.


You should have the original image infront of black now.


Group all existing groups into a new folder. Set the alpha mask for this folder.




Although this setup is tedious, you can now edit the individual parts, while previewing the final result.